Sound or audio communication in the industrial workplace has become a primary management concern, particularly in the area of voice communications. Providing information through voice broadcasts can have a direct impact on safety and production. Accordingly, a need exists for systems capable of reproducing distinguishable voice communication in an industrial environment. To this end, several sound systems have been developed which will amplify and transmit voice communications throughout a workplace. Although a multitude of environments exist in which such systems have been utilized, for purposes of the description herein it is assumed that such a system is being incorporated into a manufacturing assembly plant.
Generally sound systems designed for assembly plant use include a signal source (microphone, tape deck, etc.), an amplifier, appropriate cabling and a series of loudspeakers dispersed throughout the plant. As will be seen, the number and disbursement location of such loudspeakers is dependent upon their broadcast characteristics. The subject of the present invention encompasses only a component part of such voice communication systems, namely the loudspeaker horn. It will be noted that loudspeakers typically have attached a driver unit, for converting the amplified source signal to a sound pressure signal. The driver is usually connected to the input end of the loudspeaker soundpath. The present invention relates only to loudspeaker horn design and not to any particular driver design. As used hereinafter the term "loudspeaker" shall refer only to the horn portion and not to the driver.
The choice and consequently the design of a loudspeaker is typically based on two factors: high efficiency and coverage control or coverage angle. As used herein, high efficiency signifies a high acoustic output with low distortion and coverage angle signifies, in an ideal situation, constant directivity and beamwidth as a function of frequency for the entire intended broadcast area. Broadcast area is that area falling within an angular range, designated by the loudspeaker designer, where the speaker is positioned at the apex of the angular range. Directivity is a sound intensity ratio of the intensity of the sound wave within the intended broadcast area to the intensity of the sound wave over 360.degree..
Beamwidth plays a significant role in the description of the present invention. It will be noted that beamwidth may be generally defined as encompassing the total angle existing between those directions at which the sound pressure level (SPL) falls below 6 dB from the "head-on" axis (reference direction) SPL. SPL falling below this 6db limit has the acoustic effect of making words contained in a voice broadcast indistinguishable. Typically, speakers designated for use in an assembly plant communication system are rated for vertical and horizontal beamwidth. As can be appreciated, the greater the beamwidth, the fewer number of speakers and related equipment that will be needed to provide coverage throughout the assembly plant. As can be appreciated, to the plant coverage utilizing a minimum number of loudspeakers necessitates a design goal of maximizing beamwidth.
As indicated in Keele, D.B., What's So Sacred About Exponential Horns, Audio Engineering Society Preprint No. 1038 (F-3), pp. 1-32 (1975) an ideal horn should have constant directivity and beamwidth as a function of frequency and provide a constant acoustic load to the driver. A loudspeaker typically is constructed from one or more horns. In the real world, however, design constraints (finite size, materials, reproducible shaping) introduce various performance problems effecting beamwidth which need be minimized for particular loudspeaker applications. Presently, general categories of horns have developed, namely exponential horns of either the multi-cell or radial/sectoral type and conical horns. Keele suggests that certain of the problems associated with these types of horns can be resolved with a hybrid exponential/conical horn.
U.S. Pat. No. 4,309,932 - Keele discloses a horn having two different exponential flare surfaces oriented 90.degree. to one another. It is indicated that such a design can improve beamwidth because the precise profile of each of the two flare surfaces is achieved by a power series equation which is said to take desired beamwidth into account. However, again the largest beamwidth described in relation to that design is only 90.degree..
If one moves away from the literature to currently available loudspeakers, it can be seen that maximum beamwidths of less than approximately 100.degree. are available. Along this line three commercially available folded loudspeakers were acquired and tested. Since the present invention is directed primarily to voice communication, the subject loudspeakers were analyzed at frequencies between 500 and 5000 Hz. Utilizing available testing equipment, such as a TEF Analyzer, each loudspeaker was tested to determine loudspeaker frequency response over the 500 to 5000 Hz frequency range at locations on the reference direction and spaced 30.degree. and 60.degree. there from and polar plots were generated to determine beamwidth. All loudspeakers tested incorporated the SSD 1800 driver by Renkus-Heinz of Irvine, California. This driver is preferred because it demonstrates a reliable frequency response in the frequency range of interest.
As shown in Figures 1A-1C, the frequency response of an Atlas Sound BIA-100 Bi-Axial (BIA) loudspeaker was not so-called "flat" signal, which is the desired efficiency response, but varied significantly. As can be seen from FIGS. 2A through 2D, the BIA loudspeaker demonstrated a horizontal beamwidth between 500 and 5000 Hz within a range from approximately 20.degree. to 70.degree.. As shown in FIGS. 3A to 3D, the BIA loudspeaker exhibited a 100.degree. vertical beamwidth in the lowest frequencies but such beamwidth decreased significantly between 3000 and 5000 Hz to approximately 20.degree.. Equipment and techniques for generating polar plots of the type shown in this application are well known. It should be noted that each "ring" in the plots represents 6dB of SPL.
A second loudspeaker, a Cobraflex II B folded sectoral horn, from University Sound (IIB) was also tested. As shown in FIGS. 4A-4C, the frequency response of this loudspeaker did not yield the desired flat response signal for the subject frequency range. As can be seen in FIGS. 5A and 5D the horizontal beamwidth lies within a range of approximately 10.degree. to 70.degree. with the greatest beamwidth occurring at the lowest frequencies. Although better, the vertical beamwidth became significantly limited between 3000 and 5000 Hz the vertical beamwidth fell within the range from approximately 20.degree. to 90.degree.. It should be kept in mind that the frequency range of 500 to 5000 Hz is selected because substantially all voice communications fall within that range.
A third loudspeaker, a Cobraflex III folded sectoral horn also from University Sound (III) was tested. As shown in FIGS. 7A-7C, the frequency response of this loudspeaker although better than IIB, still was not yielding a flat response signal. As shown in FIGS. 8A-8D the horizontal beamwidth was measured to be within the range from approximately 20.degree. to 50.degree.. The vertical beamwidth again became significantly limited between 2000 and 5000 Hz. As shown in FIGS. 9A to 9D, the vertical beamwidth fell within a range from approximately 15.degree. to 75.degree.. It is also desirable to design loudspeakers having a folded soundpath. This is necessary because of horn length and environment considerations.
To achieve desired frequency response in a loudspeaker intended for broadcasting voice communications, an acoustic path of a given length is necessary. When considering space limitations of assembly plants it becomes readily apparent that the acoustic path needs to be folded. Too large a unit simply cannot be accommodated. Also, it is significantly easier for moisture to make contact with the driver in a straight horn versus a horn having a folded path. Moisture contacting the driver can result in driver short circuit and in some cases destroy the driver's transducer.
Consequently, a need exists for a folded loudspeaker which is capable of broadcasting an acoustic signal for which the sound pressure level remains within 6 dB from the "head-on" axis SPL for maximum horizontal and vertical angles.
In the present invention, a horizontal beamwidth of 120.degree. and a vertical beamwidth of 60.degree. are achieved using a new and novel loudspeaker design which also incorporates a folded soundpath. It will be noted that the folded soundpath concept is not new. For example, each of the BIA, IIB and III loudspeakers incorporate a folded soundpath or re-entrant soundpath design. Also, folded soundpaths are disclosed in U.S. Pat. Nos. 2,338,262 - Salmon and 2,751,996 - Levy.